The present invention relates to a method for separating sulfur species from hydrocarbon streams, particularly cracked naphtha streams, using extractive distillation.
Air pollution is a serious environmental problem. A major source of air pollution worldwide is the exhaust from fuel combusted in hundreds of millions of motor vehicles. Regulations have been enacted reflecting the need to reduce harmful motor vehicle emissions including nitrogen oxide (NOx) emissions through more restrictive fuel standards. The single most important factor in controlling NOx and toxic emissions is the amount of sulfur in the gasoline. In addition, fuels containing sulfur produce sulfur dioxide and other pollutants which lead to a host of environmental concerns, such as smog and related health issues, acid rain leading to deforestation, and water pollution, as well as a number of other environmental problems. In order to reduce or eliminate these environmental problems, the sulfur content of fuels has been, and will continue to be restricted to increasingly smaller concentrations, such as, for example, less than 150 or even 30 parts per million (ppm).
The problem of sulfur in fuels is compounded in many areas where there are diminishing or no domestic sources of crude oil having relatively low sulfur content. For example, in the United States the supply of domestic oil production relies increasingly on lower grade crude oil with higher sulfur content. The need for lower sulfur content fuel increases demand for imported oil having lower sulfur content, thereby increasing trade imbalance and vulnerability due to dependence on foreign sources of oil. The sulfur content in crude oil can take the form of a wide variety of both aliphatic and aromatic sulfurous hydrocarbons.
Various techniques have been developed to remove sulfur compounds from oil. One such technique, called catalytic hydrodesulfurization (HDS), involves reacting hydrogen with the sulfur compounds in the presence of a catalyst. HDS is one process within a class of processes called hydrotreating, or hydroprocessing, involving the introduction and reaction of hydrogen with various hydrocarbonaceous compounds. Hydrotreatment has been used to remove sulfur, nitrogen, and other materials such as metals, not only for environmental purposes but to avoid adverse impact on catalysts used in subsequent processing.
Cracked naphtha obtained as a product of a cracking or a coking operation may contain a significant concentration of sulfur up to as much as 13,000 ppm. Although the cracked naphtha stream constitutes approximately half of the total gasoline pool, cracked naphtha contributes a substantial quantity of undesired sulfur to the gasoline pool. The remainder of the pool typically contains much lower quantities of sulfur. The sulfur content can be decreased by (i) hydrotreating the entire feedstock to the cracking/coker unit or (ii) hydrotreating the product naphtha from these units.
Alternative (i) is a very expensive xe2x80x9cbrute forcexe2x80x9d effort that is very expensive in that it (a) requires a large hydrotreater, and (b) it consumes significant quantities of hydrogen. Alternative (ii) is a more direct approach, but unfortunately HDS of product naphtha using standard hydrotreating catalysts under conditions required for sulfur removal results in undesirable saturation of olefins. Typically olefins are present in the original feed in an amount of about 20 vol % to about 60 vol %, down to levels as low as about 2 vol %. During typical HDS of product naphtha the olefin content may be reduced, and the reduction in olefin content reduces the octane number of the product gasoline. The reduced octane number associated with desulfurization means that the fuel ultimately will need more refining, such as isomerization, blending, or other refining, to produce higher octane fuel, adding significantly to production expenses.
Selective HDS to remove sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysis, has been described in the literature.
One non-hydrotreating option for reducing sulfur in cat-naphtha streams is liquid-liquid extraction. This process separates the sulfur species from the naphtha by decantation in the liquid phase. However an unacceptably large portion of the hydrocarbons are also extracted into the solvent with sulfur species.
It would be desirable to have a process for the selective separation sulfur compounds from olefin containing fuel feedstocks, like naphtha, thereby minimizing the loss of octane value. Ideally this process would use an inexpensive procedure that is applicable under a wide range of conditions. Such a process would represent a significant advance in the art and contribute to a cleaner environment.
The present invention provides a process for separating a sulfur species from a liquid phase organic feedstream. The sulfur species have a first volatility and a remainder of the organic feedstream has a second volatility which is substantially the same as the first volatility rendering it difficult to separate the sulfur species from the remainder of the organic feedstream. The organic feedstream is contacted with a sulfur-selective solvent under extractive distillation conditions effective to decrease the first volatility to produce an overhead product comprising a lower volume percentage of the sulfur species than the feedstream, and a bottoms product comprising a higher volume percentage of the sulfur species than the feedstream.